A geographically defined population of Spanish-surnamed individuals residing in Starr County, Texas, will be the focus of a comprehensive epidemiological survey to determine the true prevalence of adult onset diabetes mellitus, and risk factors associated with its clinical complications. This will be accomplished through a population based screen measuring quantitative variables associated (1) with diabetes (hyperglycemia, glycosylated hemoglobin, GTT), and (2) with its complication (HDL-total-cholesterol, triglycerides, obesity, etc.). Diabetic probands identified through the screeing procedure will be used to ascertain families for more intesive investigations. These will center upon the study of familial aggregation of diabetes, its risk factors and complications as determined by thorough biochemical and physical examination. Pedigress for these studies will be generated through interviews and independently constructed from church and civil records. Individuals will be typed for 47 descrete biochemical and antigenic genetic markers and the combined epidemiological, medical, familial and phenotypic information analyzed for (a) the relative contribution of genetic and environmental factors to the diabetic syndrome, (b) nonrandom association of disease with particular genetic markers, (c) evidence for the existence of a major gene associated with diabetes and/or its complications, and (d) (given evidence for a genetic etiology) the existence of genetic linkage between diabetes and any polymorphic genetic marker.